The present invention relates to a golf ball material which has a good thermal stability, flow and processability, and from which high-performance golf balls endowed with excellent properties such as rebound, durability, flexibility and scuff resistance can be obtained. The invention also relates to a method for preparing such a golf ball material, and to a golf ball which includes as a component therein a molding made from such a golf ball material.
In recent years, ionomeric resins have been widely used in golf ball materials. Ionomeric resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, in which some of the acidic groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins have excellent characteristics in terms of the durability, rebound and scuff resistance of the ball.
At present, the base resins used in golf ball cover materials are generally ionomeric resins, but a variety of modifications are being made to address the constant desire by players for golf balls having a suitable degree of flexibility, a high rebound and an excellent flight performance.
For example, to improve the rebound and flow of ionomer materials, Patent Document 1 (U.S. Pat. No. 5,312,857), Patent Document 2 (U.S. Pat. No. 5,306,760), Patent Document 3 (U.S. Pat. No. 6,100,321), Patent Document 4 (U.S. Pat. No. 6,653,382) and Patent Document 5 (U.S. Pat. No. 6,777,472) describe materials composed of an ionomeric resin to which a large amount of a metallic soap (a metal salt of a higher fatty acid) has been added.
However, the metallic soap in these ionomer materials undergo decomposition and vaporization during injection molding, generating a large amount of fatty acid gases. As a result, molding defects tend to arise. Moreover, the gases that have formed deposit on the surface of the molding, markedly lowering its paintability. Moreover, when such ionomer materials are used as golf ball cover materials, because a large amount of low-molecular-weight metallic soap (a metal salt of a higher fatty acid) is included, the durability of the golf ball significantly deteriorates, making the ionomer material entirely unfit for practical use.
In addition, materials in the form of simple molten mixtures of a flexible thermoplastic resin and an ionomer have been developed (Patent Document 6: JP-A 2003-180878). However, even though such a material appears to be uniform during the production process, there has been some concern that when the material is injection molded to form a particular golf ball layer, the high shear forces in the mold will cause delamination to occur in the layer formed by the molten mixture.
An ionomer material for golf balls which has recently been developed is a homogeneous-phase, high-rebound resilience material that has an interpenetrating polymer network (IPN) structure (Patent Document 7: U.S. Patent Application No. 2004/0044136; Patent Document 8: U.S. Patent Application No. 2004/0242802; Patent Document 9: U.S. Pat. No. 7,026,399; Patent Document 10: U.S. Pat. No. 7,175,545). The ionomer material is obtained by blending a first ingredient such as an ethylene-(meth)acrylic acid copolymer with a second ingredient that is a different type of thermoplastic resin to form a resin composition, then adding a metal ionic species as a third ingredient so as to neutralize the acid on the first ingredient dispersed in the resin composition. However, in such a production method, if the different type of thermoplastic resin used as the second ingredient is poorly compatible or completely incompatible with the ionomeric resin matrix thereby created, when injection molding is carried out using these materials to form a particular golf ball layer, the high shear forces in the mold sometimes cause delamination to occur within the layer, raising concerns over a decline in the properties of the golf ball. In particular, there has been a tendency for a strong decline in the scuff resistance.    Patent Document 1: U.S. Pat. No. 5,312,857    Patent Document 2: U.S. Pat. No. 5,306,760    Patent Document 3: U.S. Pat. No. 6,100,321    Patent Document 4: U.S. Pat. No. 6,653,382    Patent Document 5: U.S. Pat. No. 6,777,472    Patent Document 6: JP-A 2003-180878    Patent Document 7: U.S. Patent Application No. 2004/0044136    Patent Document 8: U.S. Patent Application No. 2004/0242802    Patent Document 9: U.S. Pat. No. 7,026,399    Patent Document 10: U.S. Pat. No. 7,175,545